The present invention generally relates to nuclear power generation materials, systems and processes, and more particularly to the modification of uranium dioxide (UO2) nuclear fuels to increase their thermal conductivity.
Commercial light-water reactors currently use pellets of pure or burnable poison-doped uranium dioxide (UO2) as a nuclear fuel. Though uranium dioxide has demonstrated many desirable characteristics, it has a relatively low thermal conductivity that leads to the development of a large temperature gradient across the fuel pellet. This temperature gradient and the consequent high centerline temperatures limit the operational performance of a nuclear reactor due to overheating, melting, and effects such as thermal stresses that cause pellet cladding interaction and the release of fission product gases.
High fuel temperatures can be decreased and reactor performance improved by developing nuclear fuels having enhanced thermal conductivities. A high thermal conductivity nuclear fuel would decrease fuel temperatures and facilitate a reduction in pellet/cladding interaction through lessening thermal stresses that result in fuel cracking, relocation, and swelling. Additionally, fission gas release could be decreased allowing for higher fuel burn-up, and reactor safety could be improved as a result of faster thermal responses and less stored energy in the fuel pins. Ultimately, higher conductivity may also permit more energy to be generated in the reactor.
The development of enhanced thermal conductivity two-phase ceramic-metallic (cermet) nuclear fuels has been explored in the past. Such efforts have included the examination of nuclear fuels in which UO2, UO2—ThO2 or UO2—PuO2 ceramic particles are dispersed in a stainless steel or zirconium alloy matrix. Other investigated fuel forms have included aligned metal fibers dispersed in a ceramic fuel matrix. While these fuels have mainly focused on cermets that may not be chemically stable in UO2 fuels that oxidize with burnup, other options have included the development of more stable ceramic-ceramic composites.
Ceramic-ceramic (cer-cer) nuclear fuels based on uranium dioxide are also capable of having increased effective thermal conductivities. In addition to having a higher thermal conductivity than uranium dioxide, the additive ceramic must also be chemically compatible with uranium dioxide and have a comparable melting point. Silicon carbide (SiC) and beryllium oxide (BeO; beryllia) are two high melting point and high conductivity ceramics that have been considered as additives to UO2 fuels. However, SiC and UO2 exhibit chemical interactions at temperatures as low as 1200° C., and rapid reactions occur at about 1400° C. On the other hand, the UO2—BeO phase diagram shows that these two ceramic materials exist as solid equilibrium phases below about 2100° C. As such, UO2 and BeO have been considered to be excellent candidates for fabricating two-phase ceramic nuclear fuels based on UO2.
BeO has been reported to have thermal conductivities as high as 13.7 kW/m-K (at 45K) and about 370 to about 297 W/m-K (at 300K), which is about 93% that of copper at these temperatures. A high K value coupled with a high melting point and low thermal neutron absorption cross-section suggest that BeO would be an ideal material for the high conductivity phase in a nuclear fuel. In addition, BeO is close to being isotopically pure since the only naturally occurring Be isotope is Be-9 and natural oxygen is 99.8% O16. BeO has also been reported to have excellent fission product retention capabilities, and irradiation performance up to certain fast neutron and fission fragment dose or micro-cracking, when used as a ceramic matrix material. A fine particle size in fabricated fuel pellets is desirable in order to avoid micro-cracking as a result of anisotropic radiation-induced swelling that has been reported to occur in BeO ceramics.
In view of the above, the use of BeO to enhance the thermal conductivity of UO2 has been previously examined. For example, U.S. Pat. Nos. 5,180,527, 5,255,299, 5,362,426 and 5,429,775 to Hirai et al. and a technical paper authored by Ishimoto et al., Thermal Conductivity of UO2—BeO Pellet, Journal of Nuclear Science and Technology, Vol. 33, No. 2, 1996, p 134-140, presented results evidencing a net increase in thermal conductivity with BeO in a UO2-based nuclear fuel. The research appeared to demonstrate that a continuous BeO phase along UO2 boundaries can yield high thermal conductivities. On the other hand, a discontinuous BeO phase can yield conductivities that are lower by a factor of about two. To achieve a continuous BeO phase, this prior work required heating the UO2 fuel above its eutectic temperature of 2100° C., which is a process step that presents many practical problems on an industrial scale.